Examples of data communication recently performed by the use of infrared include (1) communication by remote control which is the most common one, applied to home electric appliances, and (2) communication by the use of the optical communication devices IrDa1.0 or IrDa1.1 which are standardized peripheral devices for personal computers.
Communication by remote control is one-way communication with a transmission rate of about 1 kbps, and is characterized in that a transmission distance is long (not less than 10 m). On the other hand, the optical communication device IrDA1.0 or IrDA1.1 is characterized in that though it has a short transmission distance of about 1 m, it is capable of transmission of a large quantity of data by two-way communication since it has a transmission rate of 9.6 kbps to 4 Mbps.
It is therefore necessary that the following requirements are satisfied in near future: (i) circuits should have a higher operational speed and higher precision to improve the transmission rate; (ii) devices should have high sensitivity, improved performance, and an expanded range of an operational power source voltage, to prolong the transmission distance; and, (iii) a size of a product package should be reduced. To solve problems inherent to such high-speed and high-sensitive devices and high-precision circuits, a novel technique is hereafter, a direct current removing method applied to a light receiving amplifying device in an infrared receiver and improvement of a signal to noise (S/N) ratio are discussed.
Infrared data communication is prevailing, though still being required to achieve higher speed and higher sensitivity for long-distance transmission. FIG. 7 is a circuit diagram showing an example of a light receiving amplifying device of a conventional receiver for infrared data communication use. This is a main circuit diagram of "Infrared Receiver with Variable Input Resistance for Optical Communication Systems" disclosed by the U.S. Pat. No. 5,600,128 (Date of Patent: Feb. 4, 1997).
As shown in FIG. 7, a cathode of a light receiving device 100 is connected with a power source V.sub.CC, while an anode of the light receiving device 100 is connected with a ground terminal (GND) with a load resistor R.sub.A provided therebetween. A low frequency current bypass circuit 103 is connected with the load resistor R.sub.A in parallel. The low frequency current bypass circuit 103 is composed of a circuit (1) in which a diode D.sub.A and a resistor R.sub.B are connected in series, and a circuit (2) in which a diode D.sub.B and a diode D.sub.C are connected in series, the circuits (1) and (2) being connected in parallel. The low frequency current bypass circuit 103 is connected with an amplifier 105 through a capacitor 104.
The following description will explain an operation of the foregoing circuit. In qualitative explanations, in the case where only an AC signal is present but DC light is absent, incident optical power P is converted to a light signal current by the light receiving device 100, and the light signal current I thus obtained by conversion is further converted to a voltage by the load resistor R.sub.A, thereby generating a detected voltage V.sub.B across terminals A and B. The detected voltage V.sub.B is sent through the capacitor 104 to the amplifier 105, where the voltage is amplified. If the amplifier 105 has a sufficiently high input impedance in the present conventional case, an input signal Vin to the amplifier 105 becomes an AC component of the detected voltage V.sub.B without any change.
Subsequently, the DC component of the light signal current increases, thereby causing a DC voltage of the detected voltage V.sub.B to increase. When the DC component of the detected voltage V.sub.B reaches a certain voltage level V1, the Diode D.sub.A starts operating (turned ON), thereby allowing the current to flow through the resistor R.sub.B as well. This causes a composite resistance between the terminals A and B to decrease. As a result, the incremental change in the detected voltage V.sub.B decreases, and a signal supplied to the amplifier 105 via the capacitor 104 also diminishes, causing a signal amplified by the amplifier 105 to have a smaller amplitude.
When the D.sub.C component of the light signal current further increases and reaches a voltage level V2, the diode D.sub.B and the diode D.sub.C start operating, and a composite resistance between the terminals A and B becomes substantially determined by respective ON resistance of the diodes D.sub.B and D.sub.C. Since the composite resistance between the terminals A and B thus decreases, the incremental change in the detected voltage V.sub.B further decreases, and becomes substantially constant at around the voltage level V2. Accordingly, the input signal supplied to the amplifier 105 through the capacitor 104 diminishes, thereby causing a signal obtained by amplification to have a smaller amplitude.
This is plotted in FIG. 8. As shown in FIG. 8, in the present conventional case, the resistance (composite resistance between the terminals A and B) which consequently constitutes a load is gradually lowered so that an operational range with respect to the direct current is ensured, so as to cover the incremental change in the DC component of the light signal current. Incidentally, VT in FIG. 8 represents a minimum voltage necessary for operation.
Regarding a relationship between the detected voltage and the optical signal current, the following description will explain amperage and voltage at each turning point of the relationship by using formulas, while referring to FIG. 8. In FIG. 8, a detected voltage V1 is a voltage which causes the diode D.sub.A to start operating (turned ON), and therefore it can be expressed as a rise voltage V.sub.BE of the diode D.sub.A, which is about 0.7V. A detected voltage V2 is a voltage which causes both the diodes D.sub.B and D.sub.C to start operating (turned ON), and is expressed by a sum of rise voltages of the diodes D.sub.B and D.sub.C, that is, (V.sub.BE and V.sub.BE), which is about 1.4V.
When V1 is produced across the terminals A and B as a detected voltage, a light signal current I1 is expressed as: EQU I1.apprxeq.0.7V/R.sub.A (10
Further, since current flows through both the load resistors R.sub.A and the resistor R.sub.B in the case where the light signal current is in a range of I1 to I2 in FIG. 8, a light signal current I2 is expressed as: EQU I2.apprxeq.I1+0.7/(R.sub.A .times.R.sub.B /(R.sub.A +R.sub.B))(11)
In the circuit of the conventional receiver, however, in the case where the resistance of the load resistor is set too great, the receiver becomes inferior in response, because a time constant which is a product of the resistance and an inner capacitance Cpd of the light receiving device becomes great, thereby resulting in that its response speed becomes lower so as not to follow the change of an inputted signal. Therefore, it is necessary to make the load resistor R.sub.A have a relatively small load resistance. On the other hand, in the case where the load resistor R.sub.A has a too small load resistance, there arises a problem that a thermal noise current produced by the load resistor R.sub.A cannot be suppressed. Therefore, the load resistor R.sub.A should have an adequate load resistance. Taking as an example the optical communication device IrDA1.1 that is the current infrared communication standard, to satisfy requirements regarding photo-sensitivity and response speed, the device should substantially satisfy the following conditions:
Cpd.apprxeq.25 [pF] PA1 fc.apprxeq.6 [MHz] PA1 condition of fc: 3 dB bandwidth
To achieve such response speed, the load resistance of the load resistor R.sub.A should satisfy a condition expressed by the following formula (12): EQU R.sub.A &lt;1/(2.pi..multidot.fc.multidot.Cpd).apprxeq.1.06 [k.OMEGA.](12)
Therefore, in this case, the load resistance of the load resistor R.sub.A cannot be set to or higher than 1 k.OMEGA.. On the other hand, to achieve high sensitivity for long-distance communication, it is necessary to lower the noise of the light receiving amplifying device, and to lower the noise, it is necessary to set the load resistance of the load resistor R.sub.A as great as possible. This is because a thermal noise current Inr caused due to a load resistance R, among equivalent noise referred to input of the light receiving amplifying device, is expressed as: EQU Inr=(4KT/R).sup.1/2 [A/Hz.sup.1/2 ] (13)
where K, T, and R represent Boltzmann's constant (1.38.times.10.sup.-23), absolute temperature [K], and a load resistance, respectively.
Therefore, in the case where the load resistance is 1 k.OMEGA., Inr at room temperature is found as: EQU Inr=(4.times.(1.38.times.10.sup.-23).times.300/1000).apprxeq.4.07 [pA/Hz.sup.1/2 ]
Therefore, in the above example, it is impossible to make the noise lower than 4.07. The relationship between the thermal noise current Inr and the load resistance R, expressed by the foregoing formula (13), is shown in FIG. 9.
Nowadays, however, to suppress the noise to a level of 1 [pA/Hz.sup.1/2 ] to 2 [pA/Hz.sup.1/2 ] is demanded, and therefore, improvement of the circuit is imperative. To achieve this object, usually employed is a technique wherein input impedance on the amplifier side is lowered so that the amplifier is used as transimpedance amplifier, thereby ensuring that the resistor R.sub.A of the light receiving device is made to have a great resistance. However, there is a drawback in that an operational range with respect to the DC light current is diminished when the resistor R.sub.A has a great resistance. Therefore, difficulty in ensuring the operational range of the DC light current is the greatest obstacle to clear, in improving the infrared receiver.